Christian Widener

Microstructural Evolution of 6061 Aluminum Gas-Atomized Powder and High-Pressure Cold-Sprayed Deposition

Gas-atomized 6061 aluminum powder was used as feedstock for deposition using a high pressure cold-spraying process. The microstructures of the as-received powder and cold spray processed (CSP) ultrafine-grained (UFG) 6061 depositions were characterized by different electron microscopy techniques. It was found that there is segregation of solute elements at the particle grain boundaries, which is increased after cold spraying (CS). Various microstructural features were observed in both directions (parallel and perpendicular) of the CSP layer, including low-angle grain boundaries, clustered-small-cell walls, and dislocation tangle zones. The results also indicated that a combination of different recrystallization mechanisms (i.e., continuous and geometrical) may contribute to the formation of nano and UFG structures during CS.

Review of Relationship Between Particle Deformation, Coating Microstructure, and Properties in High-Pressure Cold Spray

In the cold spray (CS) process, deposits are produced by depositing powder particles at high velocity onto a substrate. Powders deposited by CS do not undergo melting before or upon impacting the substrate. This feature makes CS suitable for deposition of a wide variety of materials, most commonly metallic alloys, but also ceramics and composites. During processing, the particles undergo severe plastic deformation and create a more mechanical and less metallurgical bond with the underlying material. The deformation behavior of an individual particle depends on multiple material and process parameters that are classified into three major groups—powder characteristics, geometric parameters, and processing parameters, each with their own subcategories. Changing any of these parameters leads to evolution of a different microstructure and consequently changes the mechanical properties in the deposit. While cold spray technology has matured during the last decade, the process is inherently complex, and thus, the effects of deposition parameters on particle deformation, deposit microstructure, and mechanical properties remain unclear. The purpose of this paper is to review the parameters that have been investigated up to now with an emphasis on the existent relationships between particle deformation behavior, microstructure, and mechanical properties of various cold spray deposits.

Annealing behaviour of 6061 aluminium deposited by high pressure cold spray

Abstract Aluminium 6061 deposited by high pressure cold spray was analysed using a transmission electron microscope (TEM) to characterise its microstructure and response to annealing to 450°C in the plane of the deposition and perpendicular to the deposited layers. The cold sprayed deposition was also analysed with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) analysis and differential scanning calorimeter (DSC). Segregation of the solute atoms (Mg and Si) at the grain boundaries during cold spraying was seen to play a crucial role in stabilising the deformation substructure until specific temperatures, and was found to have a significant effect on the annealing behaviour of the microstructure in the two different directions.

Estimating the Effect of Helium and Nitrogen Mixing on Deposition Efficiency in Cold Spray

Cold spray is a developing technology that is increasingly finding applications for coating of similar and dissimilar metals, repairing geometric tolerance defects to extend expensive part life and additive manufacturing across a variety of industries. Expensive helium is used to accelerate the particles to higher velocities in order to achieve the highest deposit strengths and to spray hard-to-deposit materials. Minimal information is available in the literature studying the effects of He-N2 mixing on coating deposition efficiency, and how He can potentially be conserved by gas mixing. In this study, a one-dimensional simulation method is presented for estimating the deposition efficiency of aluminum coatings, where He-N2 mixture ratios are varied. The simulation estimations are experimentally validated through velocity measurements and single particle impact tests for Al6061.

Friction Stir Scribe Welding of Dissimilar Aluminum to Steel Lap Joints

The use of dissimilar material combinations such as aluminum to steel has been increasing in automobile and aerospace industries due to its potential for energy savings. Achieving an acceptable joint quality with fusion welding can be problematic due to the significant differences in physical and thermal properties between materials. One alternative to conventional fusion welding is friction stir scribe welding and because of the nature of the process, it can be a viable option for joining dissimilar metal combinations. The primary emphasis of this work is to investigate the feasibility of using FSS welding as a possible option for joining 1.0 mm thick 6022 aluminum to 0.7mm electro galvanized steel sheets in a dissimilar lap weld configuration. An H13 steel pin tool featuring a tungsten carbide scribe insert was used. An investigation on the optimum size of the scribe insert was conducted to evaluate the effects of microstructure and mechanical properties.

Fabrication and Characterization of Carbon Nanotube Nanocomposites Into 2024-T3 Al Substrates Via Friction Stir Welding Process

Carbon nanotube (CNT)-aluminum (Al) nanocomposites were prepared using friction stir welding (FSW) processing, and then the mechanical properties of these nanostructured materials were determined using the universal MTS machine. The fabrication of the CNT-metal composite consisted of the following steps: (a) homogenizing the CNTs and Al powder at three different ratios: 0/100, 25/75, and 50/50, (b) compacting the mixtures into grooves that were initially machined into the substrate (2024-T3) for the three cases, (c) incorporating CNTs in a substrate by the FSW process, and (d) validating the dispersion of the CNTs into the Al substrates after the characterization steps. Scanning electron microscopy (SEM) analysis and other physical characterization tests (e.g., mechanical, metallography, and fracture surfaces) were conducted on the prepared substrates. Test results showed that CNTs were dispersed and aligned uniquely in the different locations of the metal structures depending on the FSW zones: advancing, retreating, transverse, and stir zone regions. The mechanical properties of each zone were also compared to the distribution of CNTs. The advancing side had the highest amount of CNTs mixed into the aluminum substrate while retaining the yield strength (YS); however, the elongation was reduced. The retreating side had little to no CNTs distributed into the substrate and the mechanical properties were not significantly affected. The stir zone YS had little influence of the CNTs at the lower CNT/Al powder ratio (25/50), but a significant effect was noticed at the higher ratio of 50/50. The elongation to failure was significantly affected for both cases. The transverse zone YS and elongation to failure was significantly reduced by the powder mixtures. These results may open up new possibilities in the aircraft and other manufacturing industries for future development in the field.

Structure–Properties Relations in High-Pressure Cold-Sprayed Deposits

In the cold spray (CS) process, deposits are produced by depositing powder particles at high velocity onto a substrate. Powders deposited by CS do not undergo melting before or upon impact with the substrate. This feature makes CS suitable for deposition of a wide variety of materials, most commonly metallic alloys but also ceramics and polymers or composites of those materials. During processing, the particles undergo severe plastic deformation, and both components of bonding, i.e., mechanical and metallurgical, are achieved with the underlying material, depending on the material type and impact velocity. The deformation behavior of powder particles depends on multiple material and process parameters. Changing to these parameters leads to the evolution of different microstructures and consequently changes the mechanical properties in the deposit. While CS technology has matured during the last decade, the process is inherently complex, and thus the effects of deposition parameters on particle deformation, deposit microstructure, and mechanical properties are not always clear. The purpose of this chapter is to describe the existing relationships between microstructure and mechanical properties of various CS deposits to illuminate what has been discovered to date.

Predicting the Effects of Powder Feeding Rates on Particle Impact Conditions and Cold Spray Deposited Coatings

As the industrial application of the cold spray technology grows, the need to optimize both the cost and the quality of the process grows with it. Parameter selection techniques available today require the use of a coupled system of equations to be solved to involve the losses due to particle loading in the gas stream. Such analyses cause a significant increase in the computational time in comparison with calculations with isentropic flow assumptions. In cold spray operations, engineers and operators may, therefore, neglect the effects of particle loading to simplify the multiparameter optimization process. In this study, two-way coupled (particle–fluid) quasi-one-dimensional fluid dynamics simulations are used to test the particle loading effects under many potential cold spray scenarios. Output of the simulations is statistically analyzed to build regression models that estimate the changes in particle impact velocity and temperature due to particle loading. This approach eases particle loading optimization for more complete analysis on deposition cost and time. The model was validated both numerically and experimentally. Further numerical analyses were completed to test the particle loading capacity and limitations of a nozzle with a commonly used throat size. Additional experimentation helped document the physical limitations to high-rate deposition.

Effect of Friction Stir Processing on Armor Grade Materials

The objective of this paper is to investigate the effect of friction stir processing (FSP) on the ballistic performance of armor grade aluminum and steel plates. FSP is a solid state microstructural modification process that was adapted from the concepts of friction stir welding (FSW). FSP has shown to locally refine the microstructures and eliminate many casting defects for improved mechanical and corrosion properties. Armor grade aluminum (6061 and 7039) and high-strength low-alloy (HSLA- MIL-A-12560) steel were prepared in various thicknesses. The prepared plates were then FSP in partial penetration overlapping passes to provide for the largest area of processed material possible. The plates were then tested in accordance with MIL-STD-662E to determine the V50 ballistic limit of the armor. Results from the ballistic testing of processed panels were analyzed and compared with the parent material. Microstructural differences observed between the processed and unprocessed panels were characterized and correlated with the ballistic performance.

Cold Spray Applications

This chapter will present various cold spray applications developed by the US Army Research Laboratory (ARL) associated with the repair of parts from aircraft and ships by providing actual examples and their relevance toward the advancement and implementation of cold spray technology. It is not an all-inclusive by any means, but it serves to illustrate that cold spray (CS) can be used across very diverse industry sectors. It is important to note that most cold spray applications today are for dimensional restoration of parts that have experienced wear and/or corrosion in service. Significant points of interest associated with the development of these CS applications will be highlighted. Emphasis will be placed on important aspects of process development and optimization, testing and evaluation, and qualification and specifications. The methodology and essential technical data for the transition of cold spray technology for these select applications will be discussed and illustrated by actual case studies. The substrate materials represented will include aluminum, magnesium, and titanium. The focus application will involve the restoration of magnesium aerospace components where much data will be presented, while the remaining case studies cannot be so inclusive due to page restrictions.